1. Field of the Invention
The present invention relates to a polarizer, a polarizing light source device and a liquid crystal display.
2. Description of the Related Art
Since a liquid crystal display can be small and light, it has been used in various fields. Such a liquid crystal display (10) serves to electrically change the orientation state of liquid crystal molecules in a liquid crystal cell (20), thereby controlling the polarizing state of light transmitting through the liquid crystal cell (20). The liquid crystal cell (20) usually comprises two opposed transparent electrodes, that is, a transparent electrode (21) placed on a back face side and a transparent electrode (22) placed on a front face side, and a liquid crystal layer (23) interposed therebetween (21, 22). A front face of the liquid crystal cell (20) is provided with an optical element (30) including a dichroic polarizer (31) for detecting the polarizing state of the light transmitting through the liquid crystal cell (20), a phase retarder element (32) or the like. A back face of the liquid crystal cell (20) is provided with a polarizing light source device (40) for fetching only specific polarized light and emitting the polarized light toward the liquid crystal cell (20) constituted by an optical element (50) including a dichroic polarizer (51), a phase retarder element (52) or the like which is placed on the back face of the liquid crystal cell (20), a light transmitting plate (62) having a light source (61) provided therebelow or on the side face thereof, a reflector (63) placed on the back of the light transmitting plate (62) and a diffusion sheet (70) and/or a lens sheet (71) which are/is placed between the liquid crystal cell (20) and the light transmitting plate (62) (FIG. 2). In the polarizing light source device (40), the dichroic polarizer absorbs unnecessary polarized light, thereby functioning as a filter for transmitting only necessary polarized light. Therefore, even if the dichroic polarizer is ideal, 50% of light is absorbed based on natural light set in a non-polarizing state, so that the light cannot be utilized effectively.
Therefore, Published Japanese Translation of PCT International Application No. Hei 9-507308-A has proposed a method of placing a reflective polarizer nearer to the light source than the dichroic polarizer, thereby previously reflecting polarized light in a direction of vibration which will be absorbed by the dichroic polarizer so that the reflected polarized light returns to the light source side. In such a method, light is recycled and effectively utilized so that a luminance of a picture plane of a liquid crystal display is enhanced with consuming little electricity. This publication discloses that the reflection type polarizer and the dichroic polarizer are provided in the same optical path, preferably bounded with each other, so that an optical polarizing device having a high efficiency is obtained. Furthermore, the publication discloses that an absorbance is increased so as to reduce a reflection ratio in the optical polarizing device and an absorbance of the dichroic polarizer is reduced, so that a device having a predetermined absorbance and a high transmittance can be obtained. According to this publication, an absorbance of the dichroic polarizer is preferably from 10 to 99.99%, most preferably from 50 to 99.9%, and an absorbance of a reflection type polarizer is preferably from 20 to 99.99%, most preferably from 90 to 99.9%. However, there in no disclosure about means for further enhancing the luminances of the polarizing light source device and the liquid crystal display.
One of objects of the present invention is to utilize light furthermore effectively with a polarizer, with a polarizing light source device effectively using light by a reflection type polarizer or with a liquid crystal display which enhances the luminance of a picture plane by using the polarizing light source device.
The present inventors have found that when a polarizer comprising a reflection type polarizer and a dichroic polarizer having a specific transmittance and a specific polarizing coefficient, each of which is placed so that polarizing transmission axes of these polarizers are coincident with each other on the same optical path, is used in a polarizing light source device, a brightness of the device is more increased than that of a conventional polarizing light source device. In addition, the present inventors have found that when such a polarizing light source device is utilized in a liquid crystal display, the display has a picture quality equal to or higher than that of a conventional liquid crystal display to provide a brighter picture plane. Thus, the present invention has been accomplished.
First, the present invention provides a polarizer comprising a reflection type polarizer and a dichroic polarizer, each of which is placed so that polarizing transmission axes of the reflection type polarizer and the dichroic polarizer are coincident with each other on the same optical path, wherein the dichroic polarizer has a transmittance [T(AP)(xcex)] of about 44% or more and a polarizing coefficient [P(AP)(xcex)] of about 50.0% or more.
Secondary, the present invention provide a polarizer comprising a reflection type polarizer and a dichroic polarizer, each of which is placed so that polarizing transmission axes of the reflection type polarizer and the dichroic polarizer are coincident with each other on the same optical path, wherein the dichroic polarizer has a luminous correction transmittance [Y(AP)] of about 44% or more and a luminous correction polarizing coefficient [P(AP, y)] of about 50.0% or more.
Thirdly, the present invention provide a polarizing light source device comprising, in the following order, (I) one of the polarizers described above which comprise the reflection type polarizer and the dichroic polarizer, (II) a light source and (III) a reflector, wherein the light source (II) and the reflector (III) are placed over the side of surface of the reflection type polarizer in the polarizer (I).
Fourthly, the present invention provide a polarizing light source device comprising, in the following order, (I) one of the polarizers described above which comprise the reflection type polarizer and the dichroic polarizer, (II)xe2x80x2 a light transmitting plate having a light source provided on its end and (III) a reflector, wherein the light transmitting plate (II)xe2x80x2 and the reflector (III) are placed over the side of surface of the reflection type polarizer in the polarizer (I).
Fifthly, the present invention provide a liquid crystal display comprising, in the following order, one of polarizing light source devices described above, a liquid crystal cell and another dichroic polarizer, wherein the liquid crystal cell and the dichroic polarizer are placed over the side of surface of the dichroic polarizer used in the polarizing light source device.